(i) Field of the Invention
The present invention relates to a process for preparing a film of a metal oxide or of a metal hydroxide of an element of Groups IIB or IIIA of the Periodic Table, deposited on a substrate.
(ii) Description of the Related Art
Metal oxides, in thin-film form, are very important materials in various technological fields because of their optical, electrical and catalytic properties. Among their many applications, mention may be made, for example, of the use of zinc oxide for the preparation of transparent conducting electrodes in solar cells.
The metal oxide thin films are generally obtained by vacuum deposition techniques, such as sputtering or chemical vapor deposition, or by deposition in successive layers using molecular beam epitaxy (MBE). All these processes involve expensive equipment.
Another process for preparing thin films of oxides is the reactive chemical spraying technique which is carried out in an ordinary atmosphere, without a closed chamber. However, the deposition temperatures are very high, of the order of 400-500.degree. C.
Various studies have been undertaken in order to produce deposits electrolytically. For example, Jay A. Switzer, Electrochemical Synthesis of Ceramic Films and Powders, Am. Ceram. Soc. Bull. 66, [10] 1521-24 (1987), describes the preparation of an oxide film on the anode of an electrochemical cell by oxidation of a dissolved metal ion followed by hydrolysis and calcining, the process being illustrated by the preparation of thallium oxide. This process relies on increasing the oxidation state of the metal ion in solution, with the formation, and deposition on a substrate, of an insoluble oxide. However, this process can only be implemented in order to prepare the oxide of a metal which has at least two stable oxidation states in the reaction medium. J. A. Switzer (mentioned above) and R. T. Coyle, et al., (U.S. Pat. No. 4,882,014) furthermore describe the preparation of powders of metal oxides and hydroxides as ceramic precursors. These powders are formed by precipitation near the cathode of an electrochemical cell, this precipitation being caused by the reduction of nitrate ions. Next, these powders are dried and sintered at high temperature in order to obtain the ceramic materials. The deposits possibly formed on the cathode are scraped off and recovered in powder form. The intended objective is consequently the formation of a powder, and neither the direct formation of an oxide or hydroxide film on a substrate, nor its use as such are described. Furthermore, no mention is made of an oxygen reduction reaction for the formation of an oxide or hydroxide film.